Tuning a Billion Control Loops - Automatic Control a Billion Control Loops ... Customers could with...

Tuning a Billion Control Loops

Bo Bernhardsson

Department of Automatic Control LTH,Lund University

Bo Bernhardsson Tuning a Billion Control Loops

Contents

Background

3G Mobile Phone Control Loops

Some Current Interests

Joint work with MANY other persons at Ericsson

See also “Control in Mobile Communication”, PhD course 2004,see control.lth.se


BoB

1992 PhD Automatic Control Lund

1992-93 IMA, Univ Minnesota1993-2001 Automatic Control Lund

Network ControlHybrid ControlHarmonics in Power SystemsLiquid Slosh ControlRobust Control

2000-2001 Head of the I-program

2001-2010 Ericsson Lund

2010- LU full time


BoB at Ericsson

Expert in “Mobile System Design and Optimization”

Responsible for algorithms and troubleshooting in

Physical layer signal processing

Radio/BB control algorithms

30 patent applications (15 granted so far)


Ericsson in Lund1983 Ericsson in Lund, Ideon initiated2001 BoB starts. Ericsson in Lund splits into

Sony Ericsson

Ericsson Mobile Platforms

2003-5 EMP 35 % of WCDMA world market2007-8 Delivers 90 MUnits/year2008 EMP+STNXP -> ST-EricssonNow in Lund SonyEricsson, STEricsson, Ericsson


Background




The Control Loops

Power Amplifier Control GSM

Receiver Gain Control (AGC)

Frequency Control (AFC)

Phase-lock loop control (PLL)

Power Control WCDMA

(Thermal Control)

(Antenna Weight Control)

(Control of Computational Resources)

(Timing Control LTE)


Power Amplifier Control GSM

PA single most power consuming part of radio, also possiblelarge disturbance source. SAR.

Turn on and off 2W transmitter in 28 microseconds

Must not to disturb other GSM channels (200kHz grid).

Bad control can result in very costly yield lossesBo Bernhardsson Tuning a Billion Control Loops

GSM Design Problem

Power measurement not available, only PA current measured

I vs P map varies over temp, voltage, frequency and age

Customers could with short notice change PA

Calibration costly

Remark: Resulting problem not noticeable for user of offendingphone. “6-sigma” customer specs, very tough internal reqs.


GSM Power Ramping

Choice of good ramp shape makes big difference


Receiver Automatic Gain Control (AGC)

Received power depends on distance to transmitter

WCDMA operating range between -110dBm and -25dBm

Would be extremely costly to design all blocks having >100dBdynamic range !

Dynamically adjust gains so internal levels are optimal

(Pedagogical Challenge: Find a visualization of 10−11mW!)


Automatic Gain Control

Estimate received power by low pass filtering I2 + Q2

Minimize total effect of overflow and underflow

PI-control (in logarithmic domain) with antiwindup

Gain scheduling on UE velocity

Prefer gain changes in digital blocks, minimize analog gainchange


Automatic Frequency Control

x

PLL

AFCFilter andAmpl.

Filter andAmpl.

Radio Baseband

I+jQ

−1.5 −1 −0.5 0 0.5 1 1.5−1.5

−1

−0.5

0

0.5

1

1.5

Use baseband signal to detect frequency error.

Accuracy requirement: A couple of ppb (10−9)

Fast filtering needed to follow changes in frequency error(heating, UE speed changes, BS handover).

Slow filtering needed to filter out effect of channel variationsunrelated to frequency error (e.g. fast fading)

I control + filter + gain scheduling (idle vs connected mode)


PLL Control

Ref 1/R Kd Filter VCO

1/N

outputfrequency

Output frequency = NR ⋅ fre f

G/(1+GH) 1/(1+GH)1

N/G(s)G(s)

N

ωω10

210

310

410

510

6−140

−130

−120

−110

−100

−90

−80

−70

−60

−50

Frequency [Hz]

dBc/

Hz

Phase noise spectral density

N*XTALVCO open loopPLL bandwidth=2kHz−32kHz


3G Core Problem - Power Control

WCDMA - everyone share the same frequency

Uplink (UL) and Downlink (DL) power controlled at 1500HzBo Bernhardsson Tuning a Billion Control Loops

Power Control Model

Base Station

UL QualityEstim.

UL Power

±1dB UL

DL Power

DL QualityEstim.

DL ±1dB

Mobile

Bad UL power -> Unreliable DL control -> Bad DL power

Bad DL power -> Unreliable UL control -> Bad UL power


Uplink and Downlink Power Control

Cascade control, fast inner SIR loop, slower outer quality loop

A major problem is how to estimate QoS level, for example at0.1 percent block error rate.

Antiwindup mechanisms important (since basestation has onlysmall operating power range, perhaps 25dB)

Startup, lost synchronization, change of transport formats . . .Bo Bernhardsson Tuning a Billion Control Loops

WCDMA launch and the Battle of Power Control

In 2003-2004 phone vendors and NW vendors were in heavybattles for best call statistics

Early implementations had many problems, both UE and NW

Lab testing (ala 3GPP) NOT enough to guarantee systemperformance


Battle of Power Control

Many critical issues around power control, for example around

handover (mobile connected to several base stations)loss of synchronisation and how to regain it

To get good success rates a phone could egoistically:

order many UP commands, wasting DL capacitytransmit on high UL powernot send suicide measurement reports

Problem: Only room for perhaps one such phone in each cell


UE behavior in soft handover

UEs can connect to 6 base stations. Mandated behavior:

DL power: Signals from all basestations to be combined. Acommon power control command sent to all basestations.

UL power: Or-of-down rule. Command from each basestationshould be detected separatly. If any basestation orders “down”,UE must decrease TX power !

Some early phones did not follow or-of-down rule !Bo Bernhardsson Tuning a Billion Control Loops

Power Control when Synch Lost

If synchronization is lost, try to regain synch for a while. If notpossible, close down radio connection

Mandated UE behavior

If DL synchronization is lost, turn off transmitter until synchregained

Mandated base station behavior

If UL synchronization is lost, then start ordering UPcommands

This is safe is all UEs follow the or-of-down rule

Problems when base stations didn’t follow this rule !


WCDMA Power Control Debugging

Hard to get technicalevidence, such assynchronized basestation and UE loggs

Operators wereconvinced to enforcecorrect UE behavior

1-2 years later WCDMAcall statistics werebetter than GSM

MANY other technicalissues were involved.But power control wasone of the core issues


Background




Control over a communication channel

G(z)

Channel

What Shannon channel capacity C is needed to stabilize alinear control system?

Answer: For the AWGN channel the condition is

C >∑

p

log2(ppp)

where the sum is over all unstable poles

Anytime Capacity. Work by Sahai, Mitter, Tatikonda etcBo Bernhardsson Tuning a Billion Control Loops

Communication with Feedback

+Code Decode

Figure shows energy per bit needed to communicate k bits overan AWGN channel (with error probability ǫ = 0.001)

Huge performance gain by feedback

Poor, Polyanski Verdu, IEEE Trans of Info Theory August 2011,With feedback, the Shannon bound -1.56dB/bit is obtainableeven for a block of only 1 bit


Encoder-Decoder Design

Encoder

Disturbance

Channel Decoder

MeasurementNoise

TimeDelay

AttenuatedDisturbance

Design encoder and decoder to minimize error Interpretations:

Feedforward

Real-time coding

Erik Johannesson PhD Thesis Oct 2011


Encoder-Decoder Design - Erik’s thesis

F(z)

G(z) C(z) D(z)

P(z)e

t

Choose C(z) and D(z) so that E(e2) is minimized.

Channel constraint: E(t2) ≤ σ2

Equivalent to minimize

ppR − X pp22 +1

σ2 ppX pp

21 over X ∈ H2 for some R ∈ L∞

C and D obtained from X , spectral factorization

Convex problem with nice interpretation


Event-Based Control

Minimize control action or sensor messages

When is threshold-based event generation optimal? How canoptimal threshold strategies be found?

Elliptic PDE with Neumann+Dirichlet conditions over a domainwith free boundary. Explicit solutions available for some nontrivial cases

Upcoming PhD thesis by Toivo HenningssonBo Bernhardsson Tuning a Billion Control Loops

ELLIIT - (Indoor) Navigation

Joint Channel Estimation and Positioning

Gyro, accelerometer (compass) gives local movement info

Virtual antenna array, facilitates ray identification

Cooperation with Fredrik Tufvesson EIT/Lund, PhD studentsAnders Mannesson, Atif Yaqoob


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